Data streaming through time-varying transport media

ABSTRACT

Methods of data streaming from an encoder to a decoder through a connection subjected to time-varying conditions are disclosed. The connection is assigned a nominal flow rate and an encoding coefficient associated with the connection modifies the nominal flow rate to determine a permissible flow rate compatible with a time-varying state of the connection. Multiple performance characteristics are associated with the connection and corresponding sets of performance measurements taken over adaptively selected time windows are acquired. Performance metrics having one-to-one correspondence to the performance characteristics are determined and compared with lower bounds and upper bounds of respective predefined acceptance intervals. A current encoding coefficient is computed as a function of the performance metrics and used to determine the permissible flow rate. The encoder&#39;s configuration is adapted to produce an encoded signal which maximizes signal fidelity under the constraint of the permissible flow rate.

FIELD OF THE INVENTION

The present invention relates to data communication from a source tomultiple sinks where the source may adapt its flow rate to an individualsink according to conditions of a path from the source to the individualsink.

BACKGROUND OF THE INVENTION

In a data-streaming system, a server may communicate with multiplesinks. In general, a path from a data source associated with the serverto an individual data sink comprises a first span from the source to afirst switching node in a shared network, a switched path through theshared network from the first switching node to a second switching nodeof the shared network, and a second span from the second switching nodeto the individual sink. The maximum flow rate that can be sustained by apath may vary with time according to load conditions of a shared networkand physical conditions of transmission media. Any path segment may beshared by multiple connections which may be assigned different prioritydesignations.

If a connection carries delay-tolerant data, such as computer files, theintegrity of transmitted data may be preserved, as the conditions of apath fluctuates, using known end-to-end protocols which may rely onretransmission of lost data or data perceived to be lost. If aconnection carries delay-sensitive data such as real-time video signalswhere data retransmission is not desirable, it is of paramountimportance that the signal source, or a signal encoder associated with asignal source, adapt the signal content according to a perceivedcapacity of a respective connection.

There is therefore a need for responsive means for real-timeconnection-state evaluation and signal-content adaptation to ensurepreserving service quality as connection-state varies with time.

SUMMARY OF THE INVENTION

The present invention provides methods and apparatus for controllingflow rate of data from a source to a sink according to conditions of apath from the source to the sink.

In accordance with one aspect, the present invention provides a methodof data streaming from an encoder to a decoder through a time-varyingconnection. The method comprises steps of: acquiring measurementscharacterizing the connection; estimating transmittance variation of theconnection according to the measurements; determining an adjustment of acurrent encoding rate of the encoder compatible with the transmittancevariation to attain a favorable encoding rate; and instructing theencoder to encode a signal according to the favorable encoding rate. Theconnection is initially assigned a nominal encoding rate which may benegotiated or determined according to a classification of the encoder.

The measurements may comprise performance characteristics such astransfer delay between the encoder and the decoder, proportion of dataloss, or occupancy of a receiving buffer at the decoder. Themeasurements are preferably acquired over a time window of a predefinedduration.

The encoding rate may be updated by multiplying the nominal encodingrate by a first encoding coefficient which is determined according to afirst metric derived from the measurements. The first encodingcoefficient may be determined according to a predefined function of thefirst metric. The first metric may be selected as a mean value of themeasurements over the time window.

Alternatively, the encoding rate may be updated by multiplying thenominal encoding rate by a second encoding coefficient determinedaccording to a second metric based on measurement trend during the timewindow. The encoding coefficient may be updated by multiplying itscurrent value by a factor determined according to a predefined functionof the second metric. The measurement trend may be based on a slope of aregression line relating measurements to time during the time window.The measurement trend may also be based on both the slope of theregression line and a gradient of measurements during a short periodwithin the time window.

In accordance with another aspect, the present invention provides amethod of determining an adaptable encoding rate of a signal transmittedfrom a data-streaming server to a client device through a time-varyingconnection. The method comprises steps of: determining a currentencoding rate for the connection; acquiring transfer-delay measurementsover a time window between a first time instant and a second timeinstant; and acquiring a data-loss-ratio measurement over the timewindow. A regression line relating the transfer-delay measurements torespective time instants within the time window may then be determinedand the slope of the regression line is considered to indicate a trendof the measurements. A gradient of selected transfer-delay measurementsimmediately preceding the second time instant is also determined.

A first tentative encoding rate may be determined according to the slopeand the gradient and a second tentative encoding rate may be determinedaccording to the data-loss-ratio measurement. A preferred encoding ratemay then be selected as the lesser of the first encoding rate and thesecond encoding rate, thus meeting the more stringent of thetransfer-delay and data-loss requirements.

The method further comprises a step of acquiring a measurement ofoccupancy of a buffer associated with the client device. If thepreferred encoding rate, which satisfies the transfer-delay anddata-loss performance requirements, exceeds a nominal encoding rate andthe occupancy exceeds a predefined buffer-occupancy threshold, thepreferred encoding rate is reduced to equal the nominal encoding rate inorder to avoid buffer overflow. The first tentative encoding rate may bedetermined by multiplying the current encoding rate by a first encodingcoefficient E* determined according a predefined function: E*=Γ(α,β),where α is the slope of the regression line and β is the gradient.

The method further compares the data-loss-ratio measurement, denoted θ,with a lower bound θ_(min) and an upper bound θ_(max) of a predefineddata-loss-ratio acceptance interval. If θ>θ_(max), a second encodingcoefficient E** is determined as E**=(1−θ). If θ<θ_(min), the secondencoding coefficient E** is determined as E**=χ>1, where χ is a designparameter. The current encoding rate may then be multiplied by thesecond encoding coefficient E** to determine the second encoding rate.

In accordance with a further aspect, the present invention provides asystem for data streaming. The system comprises a streaming server incommunication with a plurality of clients each client having a decoder,a data buffer, and a sink reporter.

The streaming server comprises: a signal source; an adaptable encoderfor encoding signals produced by the signal source; a source reporterfor formulating downstream control packets directed to a plurality ofclients; and a flow controller for receiving upstream control packetsfrom the plurality of clients and determining individual encodingcoefficients for the plurality of clients. Each encoding coefficientdetermines an encoding rate for a signal directed to a respectiveclient.

A sink reporter associated with a specific client formulates upstreamcontrol packets directed to the flow controller. The flow controlleruses downstream control packets and corresponding upstream controlpackets exchanged through a connection between the streaming server andthe specific client to determine a current condition of the connection.

A downstream control packet sent from the source reporter to a specificclient contains a packet identifier. A corresponding upstream controlpacket sent by a sink reporter of the specific client in response to thespecific downstream control packet contains the packet identifier.

An upstream control packet may also contain an indication of occupancyof a data buffer associated with the specific client. The flowcontroller processes the upstream control packet of the specific clientto determine an indicator of transfer delay from the streaming server tothe specific client and a proportion of lost downstream control packets.

In accordance with another aspect, the present invention provides amethod of data streaming from an encoder to a decoder through atime-varying connection. The method comprises steps of: associating anencoding coefficient with the encoder for determining a flow rate of anoutput of the encoder; defining a performance metric of the connectionand an acceptance interval of the performance metric having a lowerbound and an upper bound; acquiring a set of performance measurements ofthe connection over a time window at a current encoding coefficient;determining a current value of the metric of the connection according tothe set of measurements; and adjusting the encoding coefficient to apreferred encoding coefficient according to the current value of themetric and the acceptance interval.

If the current value of the metric is lower than the lower bound of theacceptance interval the current encoding coefficient is multiplied by afirst factor to produce the preferred encoding coefficient. If thecurrent value of the metric exceeds the upper bound, the currentencoding coefficient is multiplied by a second factor to produce thepreferred encoding coefficient. The first factor exceeds 1 and thesecond factor is less than 1. Preferably, a product of the first factorand the second factor is less than 1. If the current value of the metricis within the acceptance interval, the current encoding coefficientremains unchanged.

The first factor may be determined as a function of a difference betweenthe lower bound and the current value of the metric. The second factormay be determined as a function of a difference between the currentvalue of the metric and the upper bound. The metric may be determined asa mean value of transfer delay during the time window, a mean value ofdata loss during the time window, or an indicator of occupancy of adecoder buffer measured at the end of the time window, where the decoderbuffer holds data received through the connection.

The method further comprises a step of instructing the encoder to encodea signal according to the preferred encoding coefficient and a nominalencoding rate if the preferred encoding coefficient differs appreciablyfrom the current encoding coefficient.

In accordance with a further aspect, the present invention provides amethod of data streaming from an encoder to a decoder through atime-varying connection based on gauging multiple performancecharacteristics of the connection. The method comprises steps of:associating an encoding coefficient with the connection, where theencoding coefficient determines a flow rate of an output of the encoder;associating multiple performance characteristics with the connection;and defining multiple performance metrics having one-to-onecorrespondence to the multiple performance characteristics.

At a current encoding coefficient, multiple sets of performancemeasurements of the connection over a time window are acquired, whereeach set of performance measurements corresponds to one of the multipleperformance characteristics. A current value of each performance metricis determined using a corresponding set of measurements to produce a setof current values of performance metrics. A preferred encodingcoefficient is then determined according to the current value of eachperformance metric.

The method further adjusts the encoding coefficient by comparing currentvalues of the multiple performance metrics with respective acceptanceintervals. A set of acceptance intervals, each corresponding to one ofthe multiple performance metrics is defined. Each acceptance intervalsis defined by a respective lower bound and a respective upper bound. Thecurrent encoding coefficient is multiplied by a first factor, greaterthan 1, to produce the preferred encoding coefficient when each elementin the set of current values of performance metrics is lower than alower bound of a corresponding acceptance interval. The current encodingcoefficient is multiplied by a second factor, less than 1, to producethe preferred encoding coefficient when at least one element in the setof current values of performance metrics exceeds an upper bound of acorresponding acceptance interval.

The first factor exceeds 1 and the second factor is less than 1. Inorder to cause the encoding-coefficient to increase at a slow pace anddecrease at a relatively faster pace, the product of the first factorand the second factor may be selected to be less than 1.

The multiple performance characteristics may comprise transfer delayfrom the encoder to the decoder, data loss, and occupancy of a decoderbuffer holding data received through the connection. The multipleperformance metrics may comprise: a mean value of transfer delay duringthe time window; a mean value of data loss during the time window; and avalue of occupancy of the decoder buffer during the time window. Themultiple sets of performance measurements are acquired through exchangeof control data between the encoder and the decoder. The exchange ofdata may be based on using the real-time transport protocol (RTP) andthe real-time transport control protocol (RTCP).

In accordance with another aspect, the present invention provides amethod of determining an adaptable encoding rate of a signal transmittedfrom a data-streaming server to a client device through a time-varyingconnection. The method comprises steps of: determining a currentencoding rate for the connection; acquiring measurements of a specificcharacteristic of the connection over a time window encompassing Wmeasurements between a first time instant and a second time instant; anddetermining a metric μ of the specific characteristic from themeasurements.

If μ is within a predefined acceptance interval having a lower bound μ₁and an upper bound μ₂, the current encoding rate need not change.Otherwise, the current encoding rate may be multiplied by a factor χ₂<1if μ exceeds the upper bound μ₂ of the acceptance interval or by afactor χ₁>1 if μ is below the lower bound μ₁ of the acceptance interval.

If the specific characteristic is a transfer delay along the connection,the metric is determined according to steps of: acquiring a measurementσ of the transfer delay; updating a summation Σ by adding themeasurement σ and subtracting an entry at a current index in a circulararray V, where the circular array has W>1 entries storing previousmeasurements of the specific characteristic; and storing the measurementσ in the circular array at the current index. A counter representing acumulative number of measurements is increased by 1, and current indexis updated by adding 1 (modulo W), i.e., when the Index reaches a valueof W, the Index is reset to 0. The use of the index, modulo W,facilitates storing the most recent W measurements and the purpose ofthe counter is to space successive time windows during which metrics aredetermined. Thus, the summation Σ, used as the metric μ, is used todetermine the need for encoding-rate adjustment only if the counterequals or exceeds a threshold P*. The purpose of using the summation Σinstead of a mean value Σ/W, is to reduce the computational effort. Thelower bound μ₁ is determined as a lower bound of an acceptable transferdelay multiplied by W and the upper bound μ₂ is determined as an upperbound of an acceptable transfer delay multiplied by W. The counter isreset to zero after any adjustment of the current encoding rate. Thethreshold P* is selected to be sufficiently large so that a time gapbetween any two successive steps of adjusting the current encoding rateexceeds a predefined minimum time gap. The factor χ₁ may be determinedas a function of (μ₁−Σ) and the factor χ₂ is determined as a function of(Σ−μ₂).

If the specific characteristic is a data-loss ratio, the metric μ is adata-loss ratio θ determined over the time window. The lower and upperbounds μ₁ and μ₂>μ₁ are bounds of an acceptable data-loss ratio. Thefactor χ₁ is determined as a predetermined multiplier χ>1 when θ is lessthan μ₁, and the factor χ₂ is determined as (1−θ) for θ>μ₂.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be further described withreference to the accompanying exemplary drawings, in which:

FIG. 1 illustrates a network incorporating a system of flow-rateadaptation for connections of time-varying capacities in accordance withan embodiment of the present invention;

FIG. 2 illustrates a system comprising an encoder connecting to adecoder through a connection of variable capacity where the encodingrate is adapted to the state of the connection in accordance with anembodiment of the present invention;

FIG. 3 illustrates a circular buffer associated with the encoder of FIG.2 for storing selected measurements characterizing the connection inaccordance with an embodiment of the present invention;

FIG. 4 illustrates connection measurements received at the encoder inthe system of FIG. 2 where measurements are analyzed over disjoint oroverlapping time windows in accordance with an embodiment of the presentinvention;

FIG. 5 is a flow chart illustrating a method of determining an encodingcoefficient based on measurements variation within a time window inaccordance with an embodiment of the present invention;

FIG. 6 illustrates exemplary regression lines of successive measurementsduring time windows, where the slopes of the regression lines are usedin the method of FIG. 5 to determine preferred values of the encodingrate in accordance with an embodiment of the present invention;

FIG. 7 illustrates another set of exemplary regression lines with bothpositive and negative slopes for use in an embodiment of the presentinvention;

FIG. 8 exemplifies measurements over a time window yielding a regressionline of positive slope and a negative gradient for illustrating themethod of FIG. 5;

FIG. 9 exemplifies measurements over a time window yielding a regressionline of negative slope and a positive gradient for illustrating themethod of FIG. 5;

FIG. 10 illustrates the method of FIG. 5 where decrements of theencoding coefficient take place in discrete steps determined accordingto domains defined by both linear-regression slope and measurementgradient at window end, in accordance with an embodiment of the presentinvention;

FIG. 11 illustrates the method of FIG. 5 where increments of theencoding coefficient take place in discrete steps determined accordingto domains defined by both linear-regression slope and measurementgradient at window end, in accordance with an embodiment of the presentinvention;

FIG. 12 is a flow chart illustrating steps of acquiring connectionmeasurements using the Real-time transport protocol (RTP) and Real-timecontrol transport protocol (RTCP) in accordance with an embodiment ofthe present invention;

FIG. 13 is a flow chart illustrating processes of determining connectionperformance measurements based on RTP packets sent by an encoder andRTCP packets received from a decoder in accordance with an embodiment ofthe present invention;

FIG. 14 is a flow chart indicating steps of determining connectionmetrics and a corresponding encoding coefficient in accordance with anembodiment of the present invention;

FIG. 15 illustrates an alternative method of treating measurements takenover disjoint or overlapping time windows for determining an encodingcoefficient in accordance with an embodiment of the present invention;

FIG. 16 illustrates computation of moving-average value of measurementsover overlapping time windows for use in an embodiment of the presentinvention;

FIG. 17 illustrates a method of determining a preferred encodingcoefficient based on comparing connection metrics with predefined boundsin accordance with an embodiment of the present invention;

FIG. 18 is a flow chart illustrating basic steps of the method of FIG.17, in accordance with an embodiment of the present invention;

FIG. 19 is a flow chart providing details of the steps of the method ofFIG. 17, including further steps of minimizing computation effort inaccordance with an embodiment of the present invention;

FIG. 20 illustrates an extension of the process described with referenceto FIG. 17 for adjusting an encoding coefficient according to currentvalues of two metrics;

FIG. 21 illustrates an extension of the process described with referenceto FIG. 17 for adjusting an encoding coefficient according to currentvalues of three metrics; and

FIG. 22 illustrates variation of packet-loss ratio versus a ratio ofencoding rate of a signal transmitted by a streaming server toconnection transmittance for use in illustrating an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

Terminology

-   Encoding flow rate: The bit rate of an encoded signal may fluctuate    with time and the maximum bit rate of the encoded signal is herein    called a flow rate.-   Adaptive encoder: An adaptive encoder is a device capable of    encoding a signal to a specified flow rate within a predefined    flow-rate range. The encoding attempts to maximize encode-signal    fidelity.-   Nominal flow rate: A flow rate negotiated between a client and a    streaming server, or determined automatically by a streaming server    based on knowledge of client's equipment is herein called a nominal    flow rate.-   Encoding coefficient: An encoding coefficient, denoted E, is a    scaling factor which is multiplied by the nominal flow rate to    determine a preferred flow rate compatible with a current state of a    connection between a streaming server and a client.-   Connection transmittance: The maximum flow rate which can be    sustained by a connection from a streaming server to a client    without violating preset performance objectives is herein called a    connection transmittance.-   Performance characteristic: Performance characteristics are defined    herein as measurable connection properties such as transfer-delay    variation, data-loss proportion, signal distortion, etc.-   Scalar measurement: A connection measurement related to only one    connection property is a scalar measurement.-   Vector measurement: A number of contemporaneous connection    measurements form a vector measurement.-   Statistic: A statistic is a value (such as a mean value) derived    from a set of data samples.-   Metric: A Metric is a measure of a quality of a path or a connection    in a telecommunication network. A Metric may relate to a connection    property such as latency, reliability, capacity, etc., of a path or    connection within a path.-   Scalar metric: A metric related to one connection property is herein    called a scalar metric. A scalar metric is determined from a number    of scalar measurements.-   Vector metric: A metric related to at least two connection    properties is herein called a vector metric. A vector metric is    determined from a number of vector measurements.-   Acceptance interval: A range of metrics, between a predefined lower    bound and a predefined upper bound, considered to indicate    acceptable path or connection performance is herein called an    acceptance interval.-   Downstream control packet: A control packet sent from a streaming    server to a client is a downstream control signal.-   Upstream control packet: A control packet sent from a client to a    streaming server is an upstream control signal.-   Gradient: Conventionally, the slope of a continuous function    relating two variables is known as a gradient. In the case of a    sampled function, the gradient may be approximated by determining    the slope from a small number of samples. Herein, the gradient is    the slope of measurement samples determined over a period covering a    small number of samples.-   Regression line: A straight line drawn through a set of data and    determined according to some criterion, such as minimizing the sum    of squares of data deviation from the straight line, is a regression    line. The slope of a regression line may reliably indicate a trend    of the data if the data dispersion meets certain conditions.-   Real-time transport protocol (RTP): RTP defines a standardized    packet format for delivering audio and video over the Internet    (defined by the Internet Engineering Task Force (IETF), RFC 3550).-   Real-time transport control protocol (RTCP): RTCP provides    out-of-band control information for an RTP flow and is used    periodically to transmit control packets in a streaming session. The    primary function of RTCP is to provide feedback on connection    quality.

FIG. 1 illustrates a system for data streaming where a streaming servercomputing device 120, also to be referred to as streaming server 120,transmits data to client devices 160, also called clients 160, through ashared network 180, such as the Internet. A client 160 comprises aprocessor and a computer readable medium, it is connected to network 180through a wire-line access medium 150 or a wireless access medium 152through a base station 140 which may be connected to network 180 througha wireline or a wireless medium. In a preferred embodiment, the server120 uses existing protocols to exchange control data with clients 160.For example, the streaming server 120 may use the real-time transportprotocol (RTP) to send control data to a client 160 and the client mayuse the real-time transport control protocol (RTCP) to send control datato the streaming server. Other protocols may be devised for connectionquality control. In accordance with the present invention, a clientnegotiates a data-encoding rate with the streaming server and thestreaming server may adjust the encoding rate according to statevariation of a connection between the streaming server 120 and theclient. Notably, the streaming server 120 assigns individual encodingrates to the clients 160 according to negotiated nominal flow rates(bit-rates) and individual states of connections from the streamingserver 120 to the clients 160. The streaming server may also recognize aclient's equipment type and determine a nominal flow rate accordingly.

FIG. 2 illustrates an instance of a single connection from the streamingserver 120 to a client 160. The streaming server 120 comprises a signalsource 204, connecting to an adaptable encoder 220 through a channel206. The adaptable encoder 220 encodes a signal from source 204according to a nominal flow rate and an encoding coefficient determinedby a flow controller 246. The streaming server 120 connects to theclient 160 through a channel 260 which may have a time-varying capacity.The client 160 comprises a receiver 226 which detects information signal(e.g., a video signal) and directs the detected signal to a decoder 240which may organize the detected signal in a format suitable for a user280. A buffer 238 may be associated with the decoder 240 to hold datafor a short period of time. The detected signal may also contain controlpackets which are directed to a sink reporter 242. The sink reporter mayprocess control packets received from the source reporter 224 and sendacknowledgments and specified measurements in a control packet sent toflow controller 246 through a control channel 270.

Due to the time-varying capacity of connection 260, it is of paramountimportance that the flow controller 246 determine an accurate estimationof the transmittance of the time-varying channel 260 and compute anappropriate encoding coefficient E. The adaptable encoder 220 encodes asignal according to the nominal encoding rate assigned to the client anda current value of the encoding coefficient. The flow controller 246,comprising computer readable instructions stored in a computer readablemedium, determines permissible overall flow rate (bit-rate) orselectively adjusts the information content to preserve certainqualities of the signal. In the case of video signals, the encoder maymodify the frame rates, the content per frame, or both.

The flow controller 246 uses measurements acquired through an exchangeof control packets between the server 120 and the client 160. The sourcereporter 228, comprising computer readable instructions stored in acomputer readable medium, formulates downstream control packets (notillustrated) to be directed to clients 160. The sink reporter 242,comprising computer readable instructions stored in a computer readablemedium, associated with each client 160 formulates upstream controlpackets (not illustrated) to be sent to the flow controller 246. Theflow controller 246 uses upstream control packets and correspondingdownstream control packets exchanged through connections between thestreaming server and the clients 160 to determine current states of theconnections and appropriate individual encoding coefficients for theclients 160. Each encoding coefficient determines an encoding rate for asignal directed to a respective client.

A specific downstream control packet sent from the source reporter 228to a specific client 160 contains a downstream-control-packet identifierand an indicator of an instant of time at which the downstream packet istransmitted. A corresponding upstream control packet sent by a sinkreporter 242 of the specific client 160 in response to the specificdownstream control packet echoes the downstream-control-packetidentifier and indicates a time at which the specific client receivedthe specific downstream control packet.

The flow controller may base its computation of the encoding coefficientE on measurements received during a time window of a predefined width W.The width W may be defined in terms of a time interval, such as a fewmilliseconds, or a number of measurements acquired, for example, mostrecent 128 measurements. Hereinafter, the time window width will beexpressed in terms of a number of measurement instances.

The streaming server 120 comprises at least one processor (notillustrated) and a computer readable storage medium, comprising, forexample, non-volatile memory devices, DVD, CD-ROM, or floppy disks,having computer readable instructions stored thereon. The computerreadable instructions are executed by at least one processor to causethe adaptable encoder 220, the source reporter 228, and the flowcontroller 246 to perform the functions described hereinafter.

FIG. 3 illustrates measurements stored in a circular buffer 320maintained at the flow controller 246. For clarity, the circular bufferin the illustrated example contains only eight measurements 342 acquiredduring time instants t_(j) to t_(j+7), (j+7) being a current observationinstant. The circular buffer may hold a large number ν of records. Arecord may contain a single measurement (scalar measurement) or a set ofcontemporaneous measurements (vector measurements) together with acorresponding time indicator. A new record of index j overwrites aprevious record of index (j−ν). Thus, the circular buffer 320 retainsthe most recent ν measurements (scalar or vector measurements). Somemeasurements, such as occupancy of buffer 238 associated with a decoder240, may be extracted from upstream control packets received from client160. Additionally, flow controller 246 may determine transfer delay bycomparing a time instant at which a downstream control packet to aclient 160 is sent and a time instant at which the server 120 receives acorresponding upstream control packet from the client 160. Packet lossmay be detected by comparing sequential numbers of downstream controlpackets sent to client 160 and sequential numbers extracted fromupstream packets received from client 160. The ratio of the number oflost packets to the number of downstream control packets defined apacket-loss ratio, hereinafter denoted θ.

Each record in circular buffer 320 includes (1) a serial number 340 of adownstream control packet (denoted j, j+1, j+2, . . . , etc.), (2) atime instant 342 (denoted t_(j), t_(j+1), etc. ) at which an upstreamcontrol packet indicating a serial number 340 of the same record isreceived at the server 120; and (3) a measurement 344 included in anupstream control packet, such as occupancy of buffer 238.

Retaining sufficient serial numbers 340 enables computing packet-lossratio over a time window of arbitrary width, within reasonable bounds.Retaining sending times of downstream control packets and receivingtimes 342 of corresponding upstream control packets enables computingtransfer delay. Thus, measurements characterizing a connection mayinclude measurements calculated at flow controller 246, such as transferdelay or packet-loss ratio, and measurements 344 read directly fromupstream control packets.

A time window may be limited to cover a preset maximum number of controlpackets or a preset duration. As will be described with reference toFIG. 19, the two conditions may be used and the more stringent conditiondefines the time window.

FIG. 4 illustrates a succession of measurements extracted from controlpackets received at the flow controller 246. Metrics characterizing thetime-varying channel are computed at the end of a time window. In theexample of FIG. 4, each window covers eight measurements. The metricsmay cover some or all of several attributes such as delay, data loss,buffer occupancy at the decoder 240, and a signal quality determined byanalyzing the output of the decoder. A measurement may be a scalar,related to one connection characteristic such as data-loss proportion,or a vector covering multiple connection characteristics such astransfer delay, data loss, and occupancy of a buffer associated with theconnection.

The result of analysis of measurement data over a current window maydetermine the selection of a subsequent time window. If analysis ofmeasurements over a window results in modifying the encoding rate, i.e.,modifying the encoding coefficient E, a subsequent window may beseparated from the current window by a significant interval of time.Otherwise, the subsequent window may be adjacent to the current window;a subsequent window may also be a moving window overlapping the currentwindow.

In the example of FIG. 4, metrics of a connection determined after alast measurement of a window 420 may result in updating the encodingcoefficient. Metrics determined at the end of window 240A resulted inupdating the encoding coefficient E for the current connection.Therefore, a new window 240B immediately follows window 240A. Metricsdetermined at the end of window 240B also resulted in updating theencoding coefficient and, hence, window 240C immediately follows window240B. Metrics determined at the end of window 240C resulted in updatingthe encoding coefficient and, hence, window 240D immediately followswindow 240C. Metrics determined at the end of window 240D did not alterthe encoding coefficient. Thus, a subsequent window 240E overlaps window240D. Metrics determined at the end of window 240E did not alter theencoding coefficient. Likewise, metrics determined at the end ofoverlapping window 240F did not alter the encoding coefficient. Metricsdetermined at the end of overlapping window 240G resulted in modifyingthe encoding coefficient. Therefore a new window 240H immediatelyfollows window 240G. Metrics determined at the end of window 240Hresulted in modifying the encoding coefficient and a subsequent window240I follows immediately. None of overlapping windows (moving windows)240I, 240J, 240K, 240L, and 240M, ending in measurements labeled I, J,K, L, and M, ended in modifying the encoding coefficient. Metricsdetermined at the end of overlapping window 240N resulted in modifyingthe encoding coefficient and a new window (not illustrated) followsimmediately.

FIG. 5 illustrates basic steps for determining a preferred current valueof an encoding coefficient. In step 520, the encoding coefficient E isset to equal 1.0, i.e., the encoding rate is the nominal encoding ratedetermined when the connection is setup. It is noted that the encodernaturally produces data at rates which vary according to the nature ofthe encoded signal. The encoding coefficient E, however, causes theencoder to independently modify its output flow rate regardless of thedynamics (the fluctuating bit rate) of the encoded signal. An index ofthe circular buffer 320 (FIG. 3) is also initialized in step 520 to zero(or to any preset value not exceeding the buffer length). In step 522,the flow controller 246 acquires control data sent by sink reporter 242,extracts connection measurements from the control packets, and storesthe measurements in address “Index” of a circular buffer, after whichthe Index is increased by 1 in step 524. If the address Index reaches W,the address Index is reset to 0. If step 526 determines that the valueof Index is smaller than a window width W, step 522 is revisited toreceive and process a new measurement (scalar measurement) or a set ofconcurrent measurements (a vector measurement). The window width may beexpressed as a number of buffer records each record holding a scalarmeasurement or a vector measurement. If step 526 determines that thevalue of Index is at least equal to the window width W, step 528determines a gradient, denoted β, over a short period preceding the endof a window. If the magnitude of the gradient β is less than a firstpredefined threshold {hacek over (g)}, the encoding coefficient is notupdated and step 522 is revisited. Otherwise, step 532 determines atrend of the most recent W measurements using linear-regressiontechniques. In step 540, the magnitude of the slope, denoted α, of theregression line is compared with a second predefined threshold {hacekover (S)}. If the magnitude of α (denoted |α|) is less than {hacek over(S)}, step 522 is revisited to acquire and store a scalar measurement ora vector measurement from a new control packet received at the flowcontroller. Otherwise, if |α|≧{hacek over (S)} step 542 determines a newencoding coefficient E according to a predefined function Γ(α,β) andresets Index to 0. Step 546 ensures that the value of E is bounded to beabove a predefined minimum value E_(min) and below a predefined maximumvalue E_(max).

The value of E may exceed a preset design upper bound E_(max) iffunction Γ(α,β) allows the value of E to increase continually whenmeasurements indicate high connection transmittance. Step 546 thenreduces value of E to E_(max) (E←min(E, E_(max)).

Reducing the encoding rate below a certain value may result The value ofE may fall below a preset design lower bound E_(min) if function Γ(α,β)allows the value of E to decrease continually when measurements indicatelow connection transmittance. Step 546 then increases value of E toE_(min) (E←max(E, E_(min)).

It is noted that with negligible variations in connection transmittance,as deduced from small values of the magnitudes of β and α, step 542 maynot be activated over a considerable period of time. However, in acircular buffer 320, a new measurement overwrites an already processedprevious measurement and the slope α of the regression line isdetermined in step 532 over the most recent W scalar measurements (or Wvector measurements).

The steps of FIG. 5 are implemented by a processor associated with flowcontroller 246 according to computer-readable instructions stored in astorage medium.

FIG. 6 illustrates a succession of measurements of connectioncharacteristics such as delay, data-loss proportion, or buffer occupancyat a client 160. The slopes of regression lines 620 are positive,indicating a deteriorating connection conditions (decreasing connectiontransmittance) and, hence, a need to reduce the flow rate by reducingthe encoding rate, i.e., reducing the encoding coefficient E.

FIG. 7 illustrates a succession of measurements of connectioncharacteristics at the client 160. The slopes of regression lines 620change from a positive value to negative values indicating deteriorationfollowed by improvement of connection condition.

FIG. 8 illustrates eight scalar measurements over a window 420 of FIG.4. A regression line 620 selected to minimize the sum of squares ofdeviations from measurements, taken at time instants t_(j), t_(j+1), . .. , t_(j+7), has a slope α=1.64. The gradient β determined from the lasttwo measurements is negative. The value of E is determined from apredefined function Γ(α,β). An exemplary function Γ(α,β), represented ina tabular form, is illustrated in FIG. 10.

FIG. 9 illustrates eight scalar measurement over a window where aregression line has a slope α=−1.64. The gradient β determined from thelast two measurements is positive. The value of E determined frompredefined function Γ(α,β).

FIG. 10 illustrates an exemplary function Γ(α,β) for positive values ofthe slope α of a regression line. As indicated, the encoding coefficientE remains unchanged if the gradient β is less than 1.0 with α≧0 or ifthe gradient β is greater than or equal to 1 but α is less than 0.1. Themeasurements of FIG. 8 yield a positive regression-line slope α and anegative gradient β. Hence, according to function Γ(α,β) of FIG. 10, theencoding coefficient E remains unchanged.

Likewise, FIG. 11 illustrates the exemplary function Γ(α,β) for negativevalues of α. The value of E remains unchanged if β≧(−1.2) or if β<(−1.2)but α>(−0.2). The measurements of FIG. 9 yield a negativeregression-line slope α and a positive gradient β. Hence, according tofunction Γ(α,β) of FIG. 11, the encoding coefficient E remainsunchanged.

FIG. 12, comprising FIG. 12A and FIG. 12B, illustrate a process ofexchanging control packets between the streaming server 120 and a client160 using a known protocol. FIG. 12A illustrates a process of sending anRTP (Real-time transport protocol) control packet to a client 160 andFIG. 12B illustrates a process of receiving an RTCP (Real-time controltransport protocol) packet from the client 160 and processing thecontent of the RTCP packet in flow controller 246. The process of FIG.12A starts at step 1220 where the source reporter 228 (FIG. 2) preparesthe RTP control packet. In step 1222 a current value of the encodingcoefficient is determined. In step 1224 a sequence number of the RTPpacket and a time of transmitting the RTP packet are recorded. In step1226 the RTP packet is transmitted to a respective client 160.

The process of FIG. 12B starts at step 1240, where the flow controller246 (FIG. 2) receives the RTCP packet from sink reporter 242. In step1242 measurement data is extracted from the RTCP packet. In step 1244, adifference between a current time instant and a time instant of apreceding update of the encoding coefficient is determined. If the timedifference is less than a predefined minimum update interval, step 1240is revisited to consider a waiting or forthcoming RTCP packet.Otherwise, if the time difference equals or exceeds the minimum updateinterval, step 1246 determines a new permissible encoding rate. Step1248 whether an update of the encoding coefficient is needed. If so, anew encoding coefficient is determined. Otherwise step 1240 isrevisited. In step 1260, the encoding coefficient is updated andcommunicated to the encoder 220, and the flow controller 246 (FIG. 2) isready to consider a new RTCP packet (step 1240).

The steps of FIG. 12, further detailed in FIG. 13 and FIG. 14, areimplemented by the aforementioned processor associated with flowcontroller 246 according to computer-readable instructions stored in astorage medium.

FIG. 13 details step 1242 of FIG. 12. In step 1320, a received RTCPpacket is examined to determine if it contains a “receiver report” fromthe sink reporter 242. If the RTCP packet does not contain a receiverreport, step 1340 is implemented to determine if the RTCP packetcontains a buffer-occupancy report. If the RTCP packet contains areceiver report, step 1322 determines a transfer-delay as a timedifference between the current time of arrival of the RTCP packet andthe time of transmitting a corresponding RTP packet. The correspondingRTP packet is the RTP packet having a sequential number which matches anumber indicated in field “extended highest sequence number received” ofthe RTCP receiver report. In step 1326, a minimum transfer delay isdetermined as the lesser of the transfer delay calculated in step 1322and a previous value of the minimum transfer delay for the connection.The minimum transfer delay is initialized as an arbitrary large value.The minimum transfer delay is retained for future use as a reference forgauging fluctuating transfer delay.

FIG. 14 is a flow chart presenting an overview of steps of determiningconnection metrics and a corresponding encoding coefficient. In step1420, the slope of a regression line of performance measurements takenover a time window is computed using any of known analytical methods.The gradient of the measurements near the end of the time window is alsodetermined. In step 1422 a statistic of data-loss over the time windowis also determined. In step 1424, a preferred value of the encodingcoefficient, denoted E*, based on the regression-line slope and thegradient, is determined as described above with reference to FIGS. 10and 11. In step 1426, a preferred value of the encoding coefficient,denoted E**, is determined according to the data-loss statisticdetermined in step 1422. In step 1428, a new encoding coefficient E isselected as the lesser of E* and E**. It is noted that under favorableconnection conditions, the preferred encoding coefficient may be allowedto exceed 1.0, i.e., the encoder may produce a stream momentarily havinga flow rate (bit-rate) exceeding the nominal flow rate assigned to theconnection.

The encoding coefficient E just determined may be further modifiedaccording to occupancy of a buffer placed at the client-end of theconnection. Step 1432 directs the process to step 1260 (FIG. 12) ifbuffer-occupancy data is not available. Otherwise, step 1434 determinesif a statistic of buffer-occupancy measurements taken over the timewindow exceeds a predefined threshold. If so, and if the encodingcoefficient determined step 1428 exceeds 1.0, the preferred encodingcoefficient is reduced to one in step 1440 and the process returns tostep 1260 of FIG. 12. It is noted that steps 1434 and 1440 follow anexemplary rule. Other rules governing the use of buffer-occupancystatistics may be devised.

FIG. 15 illustrates a method of processing connection measurements,according to another embodiment, where the mean values of measurements,over successive or overlapping windows, are used to determine a newvalue of the encoding coefficient E. A statistic based on a mean valueof measurements taken over a time window 1520 and corresponding to aspecific connection characteristic, such as transfer delay or data-loss,is compared with a predefined acceptable reference value of the specificconnection characteristic. When the statistic exceeds the referencevalue by a significant amount, the encoding coefficient is reduced and,consequently, the encoding rate is decreased below the nominal flowrate. If the statistic is below the reference value by a significantamount, the encoding coefficient may be increased. Thus, two bounds μ₁and μ₂, μ₁<μ₂, corresponding to the specific characteristic, may bedefined. The encoding coefficient is increased when the statistic isbelow μ₁ and decreased when the statistic exceeds μ₂. The selection ofthe gap (μ₂−μ₁) is critical. A very small gap may result in unnecessaryrapid flapping between low values and higher values. A very large gapmay result in slow response, or even no response, to significantconnection-condition variations.

FIG. 16 illustrates a moving average defined as a mean value of scalarmeasurements, expressed in arbitrary units, taken over successiveoverlapping windows each window covering eight measurements. The meanvalues 16.25, 17.0, 16.75, and 17.75 for the first four overlappingwindows exhibit slow variation and a current value of the encodingcoefficient may remain unchanged. However, a mean value of 21.5determined after a number of measurements, as indicated in FIG. 16, maytrigger modification of the encoding coefficient and adjustingparameters of the adaptable encoder 220. As described earlier withreference to FIG. 4, metrics determined at the end of a time windowdetermine the selection of a subsequent time window.

FIG. 17 illustrates a method of attuning the encoding coefficient E to acurrent value of a metric μ of the time-varying connection 260. At agiven connection condition, the value of metric μ increases as theencoding coefficient E increases. The two bounds μ₁ and μ₂, indicated inFIG. 17 as lines 1720 and 1740 respectively, define a range ofacceptable connection performance. FIG. 17 illustrates exemplarydependence of the metric μ on the encoding coefficient E for five valuesof connection transmittance, denoted η₁, η₂, η₃, η₄, and η₅, whereη₁>η₂>η₃>η₄>η₅. Initially, E is set to equal 1 so that the encoderoperates at the nominal encoding rate allocated to a client underconsideration. The nominal encoding rate is selected so that theconnection performance is acceptable under normal connection condition,where the transmittance equals η₁. In FIG. 17, sample values of theconnection metric μ are identified by indices (0) to (9). Consider acase where the transmittance is at its maximum value η₁, when E=1.0, anda current metric has a value {index (0)}, between μ₁ and μ₂. Theconnection condition then deteriorates and the transmittance of theconnection decreases to a value η₂ leading to a new value of metric μabove the upper bound μ₂ {index (1)}. The flow controller 246 decreasesthe encoding coefficient E by a value of e₁, leading to a new value{index (2)} of metric μ within the interval (μ₁,μ₂). The connectioncondition continues to deteriorate and the connection transmittancesdecreases to η₄<η₂, leading to a new value {index (3)} of metric μ wellabove the interval (μ₁,μ₂). In response to the increased metric, theflow controller 246 reduces the encoding coefficient by e₃. The value ofthe metric μ drops {index (4)} but is still above μ₂. After two furtherreductions of the encoding coefficient by e₄ and e₅, leading to valuesrepresented by indices (5) and (6), the encoding coefficient isapproximately 0.56 and the metric μ reduces to a value {index (6)}within the interval (μ₁,μ₂). The connection conditions then improved sothat, at the same value 0.56 of the encoding coefficient E, the metric μdrops to a value {index (7)} well below the interval (μ₁,μ₂). The flowcontroller 246 increases the encoding coefficient by e₇ leading to anincreased value of μ {index (8)} which is still below μ₁. The flowcontroller 246 further increases the encoding coefficient by e₈ to avalue of 0.75 leading to an increased value {index (9)} of μ within theinterval (μ₁,μ₂). The encoding coefficient E remains at the value of0.75 until further changes in connection transmittance causes metricchanges.

FIG. 18 is a flow chart of the main steps, initially starting from step1820, of the procedure of FIG. 17. In step 1820, a new value of metric μis determined from measurements taken over a window as described abovewith reference to FIGS. 15 and 16. In step 1824, the new value iscompared with the upper bound μ₂ and if μ>μ₂ the encoding coefficient isreduced in step 1860 by multiplying a current value of E with a factorχ₂<1 and the process returns to step 1820. If μ≦μ₂, step 1826 comparesthe new value of metric μ with the lower bound μ₁ of the acceptanceinterval (μ₁,μ₂). If μ≧μ₁, it is determined that the new value of themetric is within the acceptance interval (μ₁,μ₂) and the process returnsto step 1820 to process a new value of metric μ. If μ<μ₁, the encodingcoefficient E is updated in step 1840 by multiplying a current value ofE by a factor χ₁>1 and the process returns to step 1820. The factors χ₁and χ₂, (χ₁>1, χ₂<1) may depend on the differences (μ₁−μ) and (μ₂−μ),respectively. If the product (χ₁×χ₂) is less than 1, the value of Eincreases in relatively small steps, when μ<μ₁, and decreases inrelatively large steps, E when μ>μ₂.

FIG. 19 depicts the process of FIG. 17 in further detail. The metric μis selected to be a mean value of measurements taken over a windowcovering a predefined number of measurements. To reduce thecomputational effort of the flow controller 246, the mean value isreplaced by the sum Σ of measurements over the window and the acceptanceinterval (μ₁,μ₂) is replaced by an interval (Σ₁,Σ₂), where Σ₁=w×μ₁ andΣ₂=w×μ₂, w being a number of measurements per time window.

In step 1920, the value of E is initialized at 1, i.e., the encoder isinitially requested to operate at the nominal encoding rate assigned toa client under consideration. All the entries of a circular array V(representing circular buffer 320 of FIG. 3) of length w are set toequal zero and a sum Σ of w entries in array V is initialized to equalzero. An integer “Index” tracks a current location in array V and aninteger “Period” tracks a most recent instance of encoding-coefficientupdate. Each of the two integers Index and Period is initialized in step1920 to equal zero.

In step 1922 a measurement σ is extracted from a control packet receivedat flow controller 246. In step 1924 the summation Σ, proportional tothe metric μ, is updated by subtracting an entry in location “Index” ofarray V and adding the new measurement σ. Array V is circular and,hence, after a first time window, the subtracted entry in location Indexrepresents a measurement taken before a current window. After a firsttime window, the summation Σ over the first window correctly representsthe metric μ multiplied by w.

In step 1926, a new measurement σ is written in location “Index” ofarray V. In step 1928, the integer Period is increased by 1, and thecurrent location Index of circular array V is increased by 1, modulo w,i.e., when the value of Index reaches w, Index is reset to zero. If, instep 1930, it is determined that the number of measurements since animmediately preceding instance of encoding-coefficient update is lessthan a predefined limit P*, the process returns to step 1922 to processa new measurement. Otherwise the process proceeds to step 1932. Whenstep 1932 is reached, the circular array V has w most recentmeasurements and the time interval between a current instant of a timeand the immediately preceding instant of time at which the encodingcoefficient E was modified is at least equal to a threshold P*. Thevalue of P* is selected to be sufficiently large to avoid an excessiveupdate rate and small enough to be responsive to connection statevariations. The value of P* is a design parameter set by a designer ofthe flow controller 246.

Step 1932 compares the current summation Σ with the upper bound Σ₂ ofthe acceptance summation interval (Σ₁, Σ₂). If Σ>Σ₂, E is reduced instep 1940 by multiplying its current value by a factor χ₂<1, the integerPeriod is reset to equal zero in step 1950, and the process returns tostep 1922. If Σ≦Σ₂, step 1934 compares the current summation Σ with thelower bound Σ₁ of the acceptance summation interval (Σ₁, Σ₂). If Σ<Σ₁, Eis increased in step 1942 by multiplying its current value by a factorχ₁>1, the integer Period is reset to equal zero in step 1950 and theprocess returns to step 1922. If step 1934 determines that Σ≧Σ₁, it isconcluded that the current summation Σ is within the acceptancesummation interval (Σ₁, Σ₂). The process returns to step 1922 and thevalue of E remains unchanged.

The steps of FIG. 19 are implemented by the aforementioned processorassociated with flow controller 246 according to computer-readableinstructions stored in a respective computer readable storage medium.

Multiple Connection Metrics

The metric μ considered in FIG. 17 is a scalar representing one ofmultiple aspects of connection performance, such as transfer delay, dataloss proportion, or buffer occupancy at a decoder 240. Consequently, thebounds μ₁ and μ₂ are also scalars. The criterion for modifying theencoding coefficient may be based on a connection metric μ related toone performance aspect with the factors χ₁ and χ₂, which modify theencoding coefficient (FIGS. 18 and 19), being a function of a deviationof a current value of a metric from an acceptance range of the samemetric. To take multiple aspects of connection performance into account,the corresponding metrics may be normalized and a composite metric maybe defined as a weighted sum of multiple normalized metrics with thebounds μ₁ and μ₂ selected accordingly. For example, a delay metric maybe normalized with respect some nominal delay value, such as anestimated minimum delay for the connection under consideration, thusbecoming dimensionless. A data-loss metric, which is naturallydimensionless, may be used as a normalized metric, and abuffer-occupancy metric (which is also dimensionless) may be normalizedwith respect to the capacity of a respective buffer. Consider, forexample, contemporaneous measurements of delay, data-loss proportion,and buffer occupancy of 20 milliseconds, 0.02, and 200, respectively.The delay metric of 20 milliseconds may be normalized to a value of 2.0based on a nominal (reference) delay of 10 milliseconds, and the bufferoccupancy of 200 data units may be normalized to 0.8 based on a buffercapacity of 250 data units. A composite metric μ* may be defined asμ*=μ_(delay)+a×μ_(loss)+b×μ_(buffer) where μ_(delay), μ_(loss), andμ_(buffer) denote a normalized delay-based metric, a data-loss metric,and a normalized buffer occupancy metric, respectively. Selecting theparameters a and b as 80.0 and 2.5, respectively, the composite metricμ* for the metrics of the above example is determined asμ*=2.0+80.0×0.02+2.5×0.8=5.6.

A more thorough method of considering multiple aspects of connectionperformance is to derive a separate metric and specify a separateacceptance interval for each performance aspect. When any current metricof a set of current metrics is above its acceptance interval, theencoding coefficient E is decreased. New metrics determined in asubsequent time window would be influenced by the change in encodingrate, due to change in the encoding coefficient E, and any change inconnection transmittance. The encoding coefficient E may be decreasedagain until none of the new metrics is above its respective acceptanceinterval. In contrast, the encoding coefficient E may be increased onlywhen all current metrics are below their respective acceptanceintervals. E remains unchanged when at least one of the resulting newmetrics is within its acceptance interval while none of remaining newmetrics is above its acceptance interval.

FIG. 20 illustrates the method of encoding-coefficient E adjustment,described above, according to current values of two metrics μ⁽¹⁾ andμ⁽²⁾. In FIG. 20, μ⁽¹⁾ increases upwards and μ⁽²⁾ increases downwards asindicated. The acceptance interval 2020 for the first metric, μ⁽¹⁾,herein referenced as the first acceptance interval, has a lower bound μ₁⁽¹⁾ and an upper bound μ₂ ⁽¹⁾. The acceptance interval 2040 for thesecond metric, μ⁽²⁾, hereinafter referenced as the second acceptanceinterval, has a lower bound μ₁ ⁽²⁾ and an upper bound μ₂ ⁽²⁾. Thereference numerals 2022, 2024, 2026, and 2028 indicate values of thefirst metric corresponding to encoding-coefficient values of E₁, E₂, E₃,and E₄, where E₁<E₂<E₃<E₄. The reference numerals 2042, 2044, 2046, and2048 indicate values of the second metric corresponding toencoding-coefficient values of E₁, E₂, E₃, and E₄.

With the encoding coefficient set at a value E₄, for example, the value(2028) of the first metric is determined to be above the respectiveupper bound μ₂ ⁽¹⁾ while the value (2048) of the second metric isdetermined to be within the second acceptance interval (μ₁ ⁽²⁾, μ₂ ⁽²⁾).Because the value of one of the two metrics is higher than thecorresponding acceptance upper bound, the encoding coefficient isreduced by multiplying its current value by a factor χ₂<1. The encoder220 adjusts the encoding flow rate accordingly and new values for thetwo metrics are determined from fresh measurements taken after theadjustment of the flow rate. When the encoding coefficient is reduced toa value E₃<E₄, the value (2026) of the first metric was determined to bewithin the first acceptance interval (μ₁ ⁽¹⁾, μ₂ ⁽¹⁾) while the value(2046) of the second metric was determined to be well below the lowerbound μ₁ ⁽²⁾ of the second acceptance interval (μ₁ ⁽²⁾, μ₂ ⁽²⁾). Nofurther changes to the encoding coefficient take place unless (1) theconnection state deteriorates resulting in one of the metrics to exceedsits acceptance upper bound, in which case the encoding coefficient isfurther reduced or (2) the connection state improves so that the valueof both the first and second metrics are below their respectiveacceptance lower bounds, thus providing an opportunity to increase theencoding coefficient.

With the encoding coefficient set at a value E₁, the value (2022) of thefirst metric μ⁽¹⁾ is determined to be below the lower bound μ₁ ⁽¹⁾ andthe value (2042) of the second metric is determined to be below thelower bound μ₁ ⁽²⁾. The encoding coefficient is increased to a value E₂.Consequently, the value of the second metric μ⁽²⁾ increased to a value(2044) which is within the acceptance interval (μ₁ ⁽²⁾, μ₂ ⁽²⁾) butclose to the upper bound μ₂ ⁽²⁾. The first metric μ⁽¹⁾ increased to avalue (2024) which is still below the lower bound μ₁ ⁽¹⁾. With no changein connection transmittance, a further increase of the encodingcoefficient may increase the second metric μ⁽²⁾ to a value above theupper bound μ₂ ⁽²⁾. No further changes to the encoding coefficient takeplace until the connection state changes sufficiently to either providean opportunity for increasing the encoding coefficient or forcedecreasing the encoding coefficient.

FIG. 21 illustrates the method of encoding-coefficient adjustmentaccording to current values of three metrics μ⁽¹⁾, μ⁽²⁾, and μ⁽³⁾. Threeacceptance intervals 2102, 2122, and 2142, for the first metric μ⁽¹⁾,the second metric μ⁽²⁾, and the third metric, respectively, are drawn asnon-overlapping stripes for clarity. It is understood, however, that thethree metrics, which may represent different characteristics of aconnection, are treated separately. Thus, the positions of acceptanceintervals 2102, 2122, and 2142 for metrics μ⁽¹⁾, μ⁽²⁾, and μ⁽³⁾,respectively, do not reflect their relative values. The first acceptanceinterval 2102 has a lower bound μ₁ ⁽¹⁾ and an upper bound μ₂ ⁽¹⁾. Thesecond acceptance interval 2122 for the second metric, μ⁽²⁾ has a lowerbound μ₁ ⁽²⁾ and an upper bound μ₂ ⁽²⁾. The third acceptance interval2142 for the third metric, μ⁽³⁾ has a lower bound μ₁ ⁽³⁾ and an upperbound μ₂ ⁽³⁾. Reference numerals 2104, 2106, 2108, 2110, 2112, 2114,2116, and 2118 indicate values of the first metric μ⁽¹⁾ corresponding toencoding-coefficient values of E₁ to E₇, where E_(j)<E_(j+1), 1≦j≦6.Likewise, reference numerals 2124, 2126, 2128, 2130, 2132, 2134, 2136,and 2138 indicate values of the second metric μ⁽²⁾ corresponding to E₁to E₇, and reference numerals 2144, 2146, 2148, 2150, 2152, 2154, 2156,and 2158 indicate values of the third metric μ⁽³⁾ corresponding to E₁ toE₇.

Consider a case where the adaptable encoder 220 has adjusted itsencoding parameters according to a nominal rate and an encodingcoefficient E₁. A metric vector determined at the end of a specificwindow has metric values for μ⁽¹⁾, μ⁽²⁾, and μ₍₃₎ indicated by 2104,2124, 2144. The values 2104, 2124 and 2144 are below the lower bound oftheir respective acceptance intervals. Therefore, the encodingcoefficient is increased in steps until any of the three metrics {μ⁽¹⁾,μ⁽²⁾, μ⁽³⁾}, determined from measurements taken after each step, issufficiently close an upper bound of a respective acceptance zone. Inthe example of FIG. 21, this condition is met at the value of metricμ⁽²⁾ indicated by 2126 after the adaptable encoder 220 adjusts its flowrate to correspond to an encoding coefficient E₂.

For the case where the adaptable encoder 220 has adjusted its encodingparameters according to a nominal rate and an encoding coefficient E₃,the values of metrics μ⁽¹⁾, μ⁽²⁾, and μ⁽³⁾ indicated by 2108, 2128, and2148 are below their respective acceptance intervals. The encodingcoefficient increased to a value E₄ at which the value of metric μ⁽¹⁾(reference 2110) was close to the upper bound of its acceptance intervalwhile the values of metrics μ⁽²⁾, μ⁽³⁾ (2130 and 2150) were within theirrespective acceptance intervals.

For the case where the adaptable encoder 220 has adjusted its encodingparameters according to a nominal rate and an encoding coefficient E₇,the value of metric μ⁽²⁾, indicated by 2138 exceeds the upper bound ofacceptance interval 2122. The encoding coefficient is reduced to a valueE₆<E₇. The metrics μ⁽¹⁾ and μ⁽³⁾, decreased to values 2116 and 2156below their respective acceptance intervals but metric μ⁽²⁾ decreased toa value 2136 within its acceptance interval 2122. Hence, the encodingcoefficient remained unchanged at the value E₆. The connectiontransmittance decreased resulting in a subsequent increase in themetrics to values indicated by 2114, 2134, and 2154, respectively. Withmetric μ⁽²⁾ exceeding its upper bound, the encoding coefficient isreduced to a value of E₅.

FIG. 22 illustrates packet-loss ratio θ as a function of the peakencoding rate (peak flow rate) of a signal transmitted by streamingserver 120 over a connection to a client 160. As defined earlier,connection transmittance is the peak flow rate which can be sustained bya connection from a streaming server to a client without violatingpreset performance objectives. In the example of FIG. 22, thetransmittance is defined according to the packet-loss performance only.FIG. 22 illustrates an exemplary relation between the packet-loss ratioand the peak encoding rate normalized with respect to the transmittanceof the connection. The peak encoding rate is known. However, thetransmittance of the connection may vary with time and may be unknown.If the peak encoding rate is higher than the transmittance, then thepacket-loss ratio θ is greater than zero and the ratio of the peakencoding rate to transmittance may be determined as 1/(1−θ) and theencoding rate may be reduced by a factor (1−θ) to eliminate packet loss.The packet-loss ratio may be measured over a selected time window asdescribed above. Line 2210 illustrates the packet-loss ratio θ as afunction of the ratio ρ of the peak encoding rate to currenttransmittance. The value of θ is zero for ρ≦1.0. At ρ=1.5, θ=0.33, andat ρ=2.0, θ=0.5.

If the measured value of θ is zero, then the ratio of the peak encodingrate to transmittance may be anywhere above 0 and less than 1.0, and itis difficult to accurately determine an appropriate increase of theencoding rate (i.e., an increase of the encoding coefficient E) whichwould improve signal fidelity while avoiding packet loss. The encodingrate may be increased in steps until packet loss is measured (θ>0) andthe encoding rate may then be corrected accordingly. When the measuredvalue of θ is considerably small, e.g., of the order of 0.001,encoding-rate correction may not be necessary. An acceptance intervaldefined by a lower bound θ_(min) (line 2220) and an upper bound θ_(max),(line 2230) of packet-loss ratio helps in avoiding unnecessaryprocessing for small values of θ. The values of θ_(min) and θ_(max) aredesign parameters. A value of θ_(min) of 0.001 and a value of θ_(max) of0.02 may be considered adequate.

A computer readable medium, e.g., a DVD, CD-ROM, floppy, or a memorysuch as non-volatile memory, comprising instructions stored thereon,when executed by a processor, to perform the steps of the methodsdescribed above, is also provided.

Although specific embodiments of the invention have been described indetail, it should be understood that the described embodiments areintended to be illustrative and not restrictive. Various changes andmodifications of the embodiments shown in the drawings and described inthe specification may be made within the scope of the following claimswithout departing from the scope of the invention in its broader aspect.

1. A method of determining an adaptable encoding rate of a signal transmitted from a data-streaming server to a client device through a time-varying connection, the method comprising: employing at least one processor in said data streaming-server for performing the following: determining a current encoding rate for said connection; acquiring measurements of a specific characteristic of said connection over a time window covering a number, W, of measurements; determining a metric μ of said specific characteristic from said measurements; accepting said current encoding rate conditional on a value of μ being within a predefined acceptance interval having a lower bound μ₁ and an upper bound μ₂; and adjusting said current encoding rate by one of: a factor χ₂<1 for a value of μ exceeding said upper bound μ₂ of said acceptance interval; and a factor χ₁>1 for a value of μ below said lower bound μ₁ of said acceptance interval; wherein said specific characteristic comprises a data-loss ratio and said metric is a data-loss ratio θ determined over said time window, wherein μ₁ and μ₂>μ₁ are bounds of an accetable data-loss ratio, wherein said factor χ₁ is determined as predetermined multiplier χ>1 when θ is less than μ₁, and wherein said factor χ₂ is determined as (1−θ) for θ>μ₂.
 2. The method of claim 1 wherein said specific characteristic further comprises a transfer delay along said connection and the step of determining said metric further comprises steps of: acquiring a measurement σ of said transfer delay; updating a summation Σ by adding said measurement σ and subtracting an entry at a current index in a circular array V, where said circular array has W entries storing previous measurements of said specific characteristic; storing said measurement σ in said circular array at said current index; updating said current index by adding 1, modulo W; increasing by 1 a counter representing a cumulative number of measurements; where said counter is less than a predetermined threshold P*, repeating said step of acquiring; and where said counter is at least equal to said threshold P* determining said metric as said summation Σ.
 3. The method of claim 2 wherein said lower bound μ₁ is determined as a lower bound of an acceptable transfer delay multiplied by W and said upper bound μ₂ is determined as an upper bound of an acceptable transfer delay multiplied by W.
 4. The method of claim 2 further comprising a step of resetting said counter to zero after the step of adjusting said current encoding rate.
 5. The method of claim 2 further comprising a step of determining said threshold P* so that a time gap between any two successive steps of adjusting said current encoding rate exceeds a predefined minimum time gap.
 6. The method of claim 2 wherein said factor χ₁ is determined as a function of (μ₁−Σ) and said factor χ₂ is determined as a function of (Σ−μ₂).
 7. The method of claim 1 further comprising: comparing sequential numbers of downstream control packets sent from said data-streaming server to said client device and sequential numbers extracted from upstream control packets received at said data-streaming server from said client device to detect lost packets; and determining, at said data-streaming server, said data-loss ratio as a ratio of a number of said lost packets to a number of said downstream control packets.
 8. The method of claim 1 wherein said time window is defined according to one of: specifying said number W of measurements; and specifying a time duration and setting said number W to be a total number of measurements acquired within said time duration.
 9. The method of claim 1 wherein said specific characteristic further comprises a transfer delay along said connection and the step of determining said metric further comprises steps of: determining, at said data-streaming server, said transfer delay by comparing a time instant at which said data-streaming server sends a downstream control packet to said client device and a time instant at which said data-streaming server receives a corresponding upstream control packet from said client device; and determining a moving average of said transfer delay over said time window.
 10. The method of claim 1 wherein said specific characteristic further comprises a buffer occupancy at said client device and the step of determining said metric comprises steps of: receiving buffer-occupancy indications from said client device; and determining a moving average of buffer occupancy over said time window.
 11. The method of claim 1 wherein said specific characteristic combines transfer delay, data-loss ratio, and buffer occupancy of a specific buffer at said client device and each said measurement comprises contemporaneous measurements of said transfer delay, said data-loss ratio, and said occupancy, and the step of determining said metric μ further comprises: determining a delay-specific metric and normalizing said delay-specific metric with respect to a nominal delay value to produce a normalized transfer delay μ_(delay); determining a data-loss-specific metric μ_(loss); determining a buffer-occupancy metric and normalizing said buffer occupancy with respect to a capacity of said specific buffer to produce a normalized buffer occupancy μ_(buffer); and determining said metric μ as: μ=μ_(delay)+a×μ_(loss)+b×μ_(buffer), where a and b are specified parameters.
 12. A system of data streaming configured to adjust an encoding rate of a signal transmitted from a data-streaming server over a connection to a client device, selected from among a plurality of client devices communicatively coupled to said data-streaming server, according to a condition of said connection, the system comprising a processor and a storage medium having stored thereon processor-executable instructions which cause said processor to: determine a current encoding rate for said connection; acquire measurements of a specific characteristic of said connection over a time window covering a number, W, of measurements; determine a metric μ of said specific characteristic from said measurements; accept said current encoding rate conditional on a value of μ being within a predefined acceptance interval having a lower bound μ₁ and an upper bound μ₂; and adjust said current encoding rate by one of: a factor χ₂<1 for a value of μ exceeding said upper bound μ₂ of said acceptance interval; and a factor χ₁>1 for a value of μ below said lower bound μ₁ of said acceptance interval; wherein said specific characteristic comprises a data-loss ratio and said metric is a data-loss ratio θ determined over said time window, wherein μ₁ and μ₂>μ₁ are bounds of an acceptable data-loss ratio, wherein said factor χ₁ is determined as predetermined multiplier χ>1 when θ is less than μ₁, and wherein said factor χ₂ is determined as (1−θ) for θ>μ₂.
 13. The system of claim 12 further comprising a memory device having stored thereon processor-executable instructions which cause said processor to: augment said specific characteristic to include a transfer delay along said connection; send downstream control packets to said client device; receive corresponding upstream control packets from said client device; compare a first time instant of sending each said downstream control packet and a second time instant of receiving a corresponding upstream control packet to determine transfer-delay measurements; and determine an additional metric as a moving average of said transfer-delay measurements over said time window.
 14. The system of claim 12 further comprising a memory device having stored thereon processor-executable instructions which cause said processor to: send downstream control packets to said client device; receive corresponding upstream control packets from said client device; and compare sequential numbers of said downstream control packets and sequential numbers extracted from said upstream control packets to detect lost packets; determine said data-loss ratio as a ratio of a number of said lost packets to a number of said downstream control packets.
 15. The system of claim 12 further comprising a memory device having stored thereon processor-executable instructions which cause said processor to: augment said specific characteristic to include a buffer occupancy at said client device; receive buffer-occupancy indications from said client device; and determine an additional metric as a moving average of buffer occupancy over said time window.
 16. The system of claim 12 further comprising a memory device having stored thereon processor-executable instructions which cause said processor to: select said specific characteristic as a combination of transfer delay, data-loss ratio, and buffer occupancy of a specific buffer at said client device; send downstream control packets to said client device; receive corresponding upstream control packets from said client device; compare content of said downstream control packets and corresponding upstream control packets to determine contemporaneous measurements of said transfer delay, said data-loss ratio, and said occupancy; and determine said metric according to said contemporaneous measurements over said time window.
 17. The system of claim 16 further comprising processor-readable instructions stored in said memory device which cause said processor to determine said contemporaneous measurements to include transfer-delay measurements, data-loss measurements, and measurements of occupancy of a buffer at said client device.
 18. The system of claim 12 further comprising a memory device having stored thereon processor-executable instructions which cause said processor to: determine a delay-specific metric and normalize said delay-specific metric with respect to a nominal delay value to produce a normalized transfer-delay metric μ_(delay;) determine a data-loss-specific metric μ_(loss); determine a buffer-occupancy metric and normalize said buffer occupancy with respect to a capacity of said specific buffer to produce a normalized buffer-occupancy metric μ_(buffer); and determine said metric μ as a weighted sum of said normalized transfer-delay metric, said data-loss-specific metric, and said buffer-occupancy metric, said weighted sum expressed as: μ=μ_(delay)+a×μ_(loss)+b×μ_(buffer), where a and b are specified parameters.
 19. The system of claim 12 further comprising processor-readable instructions, stored in a memory device, which cause said processor to adjust said current encoding rate at time instants separated by time gaps each exceeding a predefined minimum time gap.
 20. A non-transitory computer readable storage medium having stored thereon instructions which cause a processor to determine an adaptable encoding rate of a signal transmitted from a data-streaming server to a client device through a time-varying connection, performing steps of: determining a current encoding rate for said connection; acquiring measurements of a specific characteristic of said connection over a time window covering a number, W, of measurements; determining a metric μ of said specific characteristic from said measurements; accepting said current encoding rate conditional on a value of μbeing within a predefined acceptance interval having a lower bound μ₁ and an upper bound μ₂; and adjusting said current encoding rate by one of: a factor χ₂<1 for a value of μ exceeding said upper bound μ₂ of said acceptance interval; and a factor χ₁>1 for a value of μ below said lower bound μ₁ of said acceptance interval; wherein said specific characteristic comprises a data-loss ratio and said metric is a data-loss ratio θ determined over said time window, wherein μ₁ and μ₂>μ₁ are bounds of an acceptable data-loss ratio wherein said factor χ₁ is determined as predetermined multiplier χ>1 when θ is less than μ₁ and wherein said factor χ₂ is determined as (1−θ) for θ>μ₂. 